Remove harmful attempts to be helpful in pthread_mutex functions.

Most callers won't check for EINVAL, so it's best to fail early.
GCC takes the nonnull attribute as a guarantee that an argument
won't be NULL, so these hacks were already ineffective, which is
how we found that at least one commercial game was using NULL
as if it's a mutex, but actually getting no-op behavior.

Bug: 11971278
Change-Id: I89646e043d931778805a8b692e07a34d076ee6bf