Remove init_count zeroing from PAL cleanup

Remove init_count zeroing from PAL cleanup

Setting the init_count to 0 in the PALCommonCleanup was causing
intermittent crashes in the GC stress C tests on Linux with
SIGILL. The reason is that the signal handlers do nothing and call a
previous handler in case the PAL is not initialized, which is indicated
by the init_count being zero.

To fix that issue, I have removed the init_count zeroing at process
exit, since the PAL and runtime is still capable of handling the
signals.

Commit migrated from https://github.com/dotnet/coreclr/commit/638b1c1eeaf49282dcb036376ae6e35a7b62aee3