Remove obsolete mono patch for pthread_mutexattr_setprotocol

This has already been fixed in Mono both master and 2019-06 (no other branch other than the skipped 2019-04 branch uses pthread_mutexattr_setprotocol).

modules/mono/build_scripts/patches/fix-mono-android-pthread_mutexattr_setprotocol.diff

@@ -1,13 +0,0 @@
-diff --git a/mono/utils/mono-os-mutex.h b/mono/utils/mono-os-mutex.h
-index e8039bf4094..ee39c0330b3 100644
---- a/mono/utils/mono-os-mutex.h
-+++ b/mono/utils/mono-os-mutex.h
-@@ -57,7 +57,7 @@ mono_os_mutex_init_type (mono_mutex_t *mutex, int type)
- 	if (G_UNLIKELY (res != 0))
- 		g_error ("%s: pthread_mutexattr_settype failed with \"%s\" (%d)", __func__, g_strerror (res), res);
- 
--#ifdef PTHREAD_PRIO_INHERIT
-+#if defined(PTHREAD_PRIO_INHERIT) && __ANDROID_API__ >= 28
- 	/* use PTHREAD_PRIO_INHERIT if possible */
- 	res = pthread_mutexattr_setprotocol (&attr, PTHREAD_PRIO_INHERIT);
- 	if (G_UNLIKELY (res != 0 && res != ENOTSUP))