[libc] Delete teardown_main_tls and all its uses (#174374)

teardown_main_tls is called right before the main process exit to unmap
the TLS but that's unnecesary since the process is terminating anyway,
this just introduces extra system call. This can be potentially harmful
in case one of the other threads tries the access the memory that has
been unmapped. Other C library implementations don't unmap TLS either so
this makes LLVM libc's behavior more consistent.

Fixes #173409

GitOrigin-RevId: f54dd47e1d2c32d028820cd54bcb04a5cb82962d