bug#5800: remove LIBS_DEBUG and C_DEBUG_SWITCH

[Dan Nicolaescu <dann@tensilica.com>]

src/Makefile.in has this:

/* Unless inhibited or changed, use -lg to link for debugging.  */
#ifndef LIBS_DEBUG
#define LIBS_DEBUG -lg
#endif

All ports that bother to define LIBS_DEBUG just use:
#define LIBS_DEBUG

-lg has not been in use for a long time so removing it will do no harm.

Also any reason not to remove the C_DEBUG_SWITCH knob?
If users want to debug emacs, they can edit the Makefile directly, no
reason to add yet another variable to the build system.