shared: remove weston_config_get_libexec_dir()

Now that WESTON_MODULE_MAP supersedes WESTON_BUILD_DIR for libexec
binaries, we don't need to check in WESTON_BUILD_DIR anymore.

There was only one user of weston_config_get_libexec_dir(), so remove
the whole function. There is no reason to export it.

Due to libshared.la being pulled into libweston, this probably was
libweston ABI unintended. Regardless, libweston major has already been
bumped.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emre Ucan <eucan@de.adit-jv.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>