[libc++][NFC] Add reminder to remove _LIBCPP_HAS_EXPLICIT_THIS_PARAMETER macro (#127070)
`__cpp_explicit_this_parameter` has been set in LLVM20
https://github.com/llvm/llvm-project/pull/107451 /
https://github.com/llvm/llvm-project/commit/72e58e00c27c7f4dd9502c719ddd13dcfef33e85
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Co-authored-by: Hristo Hristov <zingam@outlook.com>