Fix EXTRA_DIST for applet gschemas

The following fails to include the needed schemas files:
$(gsettings_SCHEMAS_in_in)
replacing it with:
$(gsettings_SCHEMAS).in.in
Remove MAINTAINERS (does not exist) file from extra dist
Replace old gconf with gsettings schema
Remove Invest_Applet.xml from po/POTFILES.in

It causes gettext to throw a fit and mateweather applet has a simular
file and it is also not in POTFILES.in

The error it throws is:
junk after document element at line 2, column 0, byte 45 at
/usr/lib64/perl5/vendor_perl/5.16.3/x86_64-linux/XML/Parser.pm line 187.
/usr/bin/xgettext: error while opening
"../invest-applet/data/Invest_Applet.xml.h" for reading: No such file or
directory
ERROR: xgettext failed to generate PO template file. Please consult
       error message above if there is any.
Mixer applet moved, correct POTFILES.in
Mixer applet moved so also remove MATE_MixerApplet.server.in
Fix make distcheck

Just use CLEANFILES instead of DISTCLEANFILES
Correct name, server -> service
Add applet_DATA to CLEANFILES