Remove old .gitignore from Jansson

This was causing jansson_config.h to be excluded when the repo was
cloned with certain git pull commands.

src/jansson/.gitignore

@@ -1,27 +0,0 @@
-*~
-*.o
-*.a
-.libs
-.deps
-Makefile
-Makefile.in
-aclocal.m4
-autom4te.cache
-config.guess
-config.h
-config.h.in
-config.log
-config.status
-config.sub
-configure
-depcomp
-install-sh
-libtool
-ltmain.sh
-missing
-*.lo
-*.la
-stamp-h1
-*.pyc
-*.pc
-/src/jansson_config.h