Remove obsolete comment and code from fe-auth.c.

We don't use getpwuid() anymore (see commit e757cdd6), so we don't need
locking around pg_get_user_name().

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA%2BhUKGLtmexrpMtxBRLCVePqV_dtWG-ZsEbyPrYc%2BNBB2TkNsw%40mail.gmail.com

src/interfaces/libpq/fe-auth.c

@@ -1179,15 +1179,6 @@ pg_fe_getusername(uid_t user_id, PQExpBuffer errorMessage)
 	char		pwdbuf[BUFSIZ];
 #endif
 
-	/*
-	 * Some users are using configure --enable-thread-safety-force, so we
-	 * might as well do the locking within our library to protect getpwuid().
-	 * In fact, application developers can use getpwuid() in their application
-	 * if they use the locking call we provide, or install their own locking
-	 * function using PQregisterThreadLock().
-	 */
-	pglock_thread();
-
 #ifdef WIN32
 	if (GetUserName(username, &namesize))
 		name = username;
@@ -1209,8 +1200,6 @@ pg_fe_getusername(uid_t user_id, PQExpBuffer errorMessage)
 			libpq_append_error(errorMessage, "out of memory");
 	}
 
-	pgunlock_thread();
-
 	return result;
 }