Remove the dependency on WTL

[yukawa]

Description

As of 7a5226f0617ce241b4e9da5d81bff79e46ae170a, src/third_party/wtl is the only remaining 3rd party component that is checked in as a local copy rather than git submodules.

By removing the dependency on WTL, we can

• remove one more external library, which is not actively developed/maintained these days
• simplify our bzlmod migration process that is supposed to happen in a near time future

Version or commit-id

7a5226f0617ce241b4e9da5d81bff79e46ae170a

[yukawa]

Done with affc4ad6481fe444a866d4c7e9ec4788588931b9.