Remove 'remove_component_from_entity'

Without this commit, there was code using two different methods to
remove components from the document. Both types of component (element
or attribute) inherit from the same parent in REXML. They both have a
function to remove themselves from the document. There was no need to
call different removals for different types.

This commit removes the 'remove_component_from_entity' method and
simplifys the logic in 'apply_component'.