Remove dbx.h, do not set PREFERRED_DEBUGGING_TYPE from dbxcoff.h, lynx.h

Remove dbx.h, do not set PREFERRED_DEBUGGING_TYPE from dbxcoff.h, lynx.h

The following removes the unused config/dbx.h file and removes the
setting of PREFERRED_DEBUGGING_TYPE from dbxcoff.h which is
overridden by all users (djgpp/mingw/cygwin) via either including
config/i386/djgpp.h or config/i386/cygming.h

There are still circumstances where mingw and cygwin default to
STABS, namely when HAVE_GAS_PE_SECREL32_RELOC is not defined and
the target defaults to 32bit code generation.

The new style handling DBX_DEBUGGING_INFO is in line with
dbxelf.h which does not define PREFERRED_DEBUGGING_TYPE either.

The patch also removes the PREFERRED_DEBUGGING_TYPE define from
lynx.h which always follows elfos.h already defaulting to DWARF,
so the comment about STABS being the default is misleading and
outdated.

2021-09-09  Richard Biener  <rguenther@suse.de>

	PR target/102255
	* config/dbx.h: Remove.
	* config/dbxcoff.h: Do not define PREFERRED_DEBUGGING_TYPE.
	* config/lynx.h: Likewise.