Replace alloc() with xmalloc() and remove immediate OOM checks

src/nvim/memory.c

@@ -44,14 +44,6 @@
 
 static void try_to_free_memory();
 
-/*
- * Note: if unsigned is 16 bits we can only allocate up to 64K with alloc().
- */
-char_u *alloc(unsigned size)
-{
-  return xmalloc(size);
-}
-
 /// Try to free memory. Used when trying to recover from out of memory errors.
 /// @see {xmalloc}
 static void try_to_free_memory()