core,libnm: use nm_clear_g_source() where possible

core,libnm: use nm_clear_g_source() where possible

Replacement was done with commands:

spatch --sp-file nm_clear_g_source.cocci --in-place --smpl-spacing --dir src
spatch --sp-file nm_clear_g_source.cocci --in-place --smpl-spacing --dir libnm

where nm_clear_g_source.cocci contains:

@@
expression e;
@@
- if (e) {
-    g_source_remove (e);
-    e = 0;
- }
+ nm_clear_g_source (&e);