Use official absl instead of s2/absl and clean

[shangfu-peng]

Following https://github.com/jmr/s2geometry/commit/a4884a3b023180290b0d5fa24a4d4059fb5746c8 to use the offcial absl

1. Add workspace and build
2. remove s2/absl, and use the com_google_absl
3. Remove s2/btree to use the absl/btree
4. some cleans

bazel build works now.

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