Remove <pthread.h> cruft.

The next NDK to take these headers only supports API 21 and later.

Note that this change leaves the _implementation_ of these functions
behind, so that any old apps calling these APIs should continue to work,
you just can't (without declaring the functions yourself) write new ones
that do (and declaring the functions yourself would only work on LP32
anyway, so that's not going to get you very far in 2023).

Test: treehugger
Change-Id: Ie03514e4215b40f6e9feaa6e4bf5df5b16dc8d59
diff --git a/libc/bionic/pthread_mutex.cpp b/libc/bionic/pthread_mutex.cpp
index a15e981..9b37225 100644
--- a/libc/bionic/pthread_mutex.cpp
+++ b/libc/bionic/pthread_mutex.cpp
@@ -945,6 +945,8 @@
 }
 
 #if !defined(__LP64__)
+// This exists only for backward binary compatibility on 32 bit platforms.
+// (This function never existed for LP64.)
 extern "C" int pthread_mutex_lock_timeout_np(pthread_mutex_t* mutex_interface, unsigned ms) {
     timespec ts;
     timespec_from_ms(ts, ms);