i18npool, forms: remove warnings_guard_unicode*.h

These horrible headers use "#pragma GCC system_header" and thereby
prevent generated Makefile dependencies, which is what broke incremental
builds after the ICU upgrade today.

Change-Id: Ife983b3c4de86968da8a0187a1acb1bbb2b5c81f
ucb: kill webdav-neon/warnings_guard_ne_locks.h too

Change-Id: I44919cf592eee026116fc5fbc12e0d5baa4bda2b
remove more pragma instances of GCC system_header

Use "#pragma GCC diagnostic ignored" instead which does not prevent
generation of dependencies.

Change-Id: Ic9034f20ef71c1ee6e5363b73c831142c2813d20