res_musiconhold: Don't chdir() when scanning MoH files

There doesn't appear to be any reason that we are chdir'ing in
moh_scan_files, and in the event of an Asterisk crash, the core files
may not get written because we have changed into a read-only directory.

ASTERISK-23996 #close
Reported by: Walter Doekes

Change-Id: Iac806dce01b3335963fbd62d4b4da9a65c614354