Revert "posix: procN: add missing alias for getpid()"

This reverts commit be086f1.

PR zephyrproject-rtos#73978 introduced a regression.
Unfortunately this PR cannot be reverted without reverting also
Let's revert both PRs to stabilize main again towards the 3.7 release.

For more details on the issue see
zephyrproject-rtos#75205

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

lib/posix/options/multi_process.c

@@ -21,6 +21,3 @@ pid_t getpid(void)
 
 	return 42;
 }
-#ifdef CONFIG_POSIX_MULTI_PROCESS_ALIAS_GETPID
-FUNC_ALIAS(getpid, _getpid, pid_t);
-#endif /* CONFIG_POSIX_MULTI_PROCESS_ALIAS_GETPID */