chore: delete some unused functions in meta.zig

[190n]

What does this PR do?

Removes these functions from src/meta.zig:

• OptionalChild: only used in one place, unclear name (it required a pointer to an optional, not just an optional). Removed and its user was replaced with @TypeOf
• ReturnOfMaybe, MaybeResult: not used
• ConcatArgs{1,2,4}: only the 2 variant was used in two places. Those uses have been replaced by tuple concatenation.
• CreateUniqueTuple: not used
• hasStableMemoryLayout: not used, and wrong (auto-layout structs and unions never have stable layout; not all ints have stable layout ziglang/zig#19755)
• isScalar: not used, and wrong (8- and 16-bit integers are scalars)

How did you verify your code works?

Checked that it still compiles

[robobun]

:x: @190n, your commit 927cb549dfe0a9d28d215ef9c32e7b50fe00faf0 has 11 failures in #6403:

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64-baseline

test/cli/install/registry/bun-install-registry.test.ts - timeout on 🐧 3.20 aarch64

test/cli/install/registry/bun-install-registry.test.ts - timeout on 🐧 3.20 x64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 aarch64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64-baseline

test/v8/v8.test.ts - 23 failing on 🐧 20.04 x64-baseline

test/js/web/fetch/fetch.stream.test.ts - 1 failing on 🍎 14 x64

test/js/web/fetch/fetch.stream.test.ts - 1 failing on 🐧 3.20 x64

test/js/bun/ffi/cc.test.ts - 1 failing on 🐧 3.20 aarch64

test/js/bun/ffi/cc.test.ts - 1 failing on 🐧 3.20 x64

test/js/bun/ffi/cc.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 aarch64

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 aarch64

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 x64

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 x64-baseline

test/js/bun/http/serve.test.ts - 1 failing on 🪟 2019 x64-baseline

test/js/bun/http/serve.test.ts - 1 failing on 🍎 13 x64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 aarch64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 x64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 x64-baseline

test/js/node/test/parallel/child-process-exec-timeout-kill.test.js - 1 failing on 🍎 14 aarch64

test/cli/run/require-cache.test.ts - 1 failing on 🪟 2019 x64-baseline

test/cli/run/require-cache.test.ts - 1 failing on 🪟 2019 x64