Low: libcrmcommon: Always use XML substitutions

This way, substitutions will take effect when the API functions are
called directly instead of via certain CLI tools. It's not clear why
--xml-substitute was made an option in the first place instead of the
substitutions being used all the time. However, --xml-substitute has
been a hidden option.

See this thread:
https://github.com/ClusterLabs/pacemaker/pull/3308#discussion_r1469877724

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>