clang compiler warnings: Fix -Wunused-function; make inline function static

This patch fixes clang compilers warnings for unused functions. Specifically:
 * channels/chan_iax2: removed user_ref function
 * main/dsp.c: removed goertzel_update function
 * main/config.c: made variable_list_switch static

Review: https://reviewboard.asterisk.org/r/4527

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4527.patch submitted by dkdegroot (License 6600)
........

Merged revisions 433678 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 433680 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433681 65c4cc65-6c06-0410-ace0-fbb531ad65f3