make_xml_documentation: Remove usage of get_sourceable_makeopts

get_sourceable_makeopts wasn't handling variables with embedded
double quotes in them very well.  One example was the DOWNLOAD
variable when curl was being used instead of wget.  Rather than
trying to fix get_sourceable_makeopts, it's just been removed.

ASTERISK-29986
Reported by: Stefan Ruijsenaars

Change-Id: Idf2a90902228c2558daa5be7a4f8327556099cd2