parse_profile_data() returns the parsed profiles, but writes to
existing_profiles directly.

read_profiles() calls parse_profile_data() and already handles adding
the parsed profiles to aa, original_aa or extras, which means updating
existing_profiles there is a much better place.

This commit also includes a hidden change: Previously, when parsing
include files, they were also added to existing_profiles. This is
superfluous, only real profiles need to be stored there.