Rewrite guide: deduplicate front-controller/FallbackResource content

Rewrite guide: deduplicate front-controller/FallbackResource content

remapping.xml had two sections (front-controller and fallback-resource)
that duplicated the advice in avoid.xml. Replace front-controller with
a brief cross-reference to avoid.xml and htaccess.xml; remove the
fallback-resource section entirely (it also referenced pre-2.2.16
Apache, which is no longer relevant).

Update htaccess.xml cross-reference link to point to
avoid.html#fallback-resource (its canonical home).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1934084 13f79535-47bb-0310-9956-ffa450edef68