gdb/breakpoint: display "N" on MI for disabled-by-condition locations

gdb/breakpoint: display "N" on MI for disabled-by-condition locations

For breakpoint locations that are disabled because of an invalid
condition, CLI displays "N*" in the 'enabled' field, where '*' refers
to the footnote below the table:

  (*): Breakpoint condition is invalid at this location.

This is not necessary for MI, where we shall simply print "N" without
the footnote.

Update the document to mention the "N" value for the MI.  Also remove
the line about the 'enable' field, because there is no such field for
locations.

gdb/ChangeLog:
2021-04-21  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* breakpoint.c (print_one_breakpoint_location): Display "N" for
	disabled-by-condition locations on MI-like output.
	(breakpoint_1): Do not display the disabled-by-condition footnote
	if the output is MI-like.

gdb/doc/ChangeLog:
2021-04-21  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* gdb.texinfo (GDB/MI Breakpoint Information): Update the
	description for the 'enabled' field of breakpoint locations.