Revert "Add completion styling"

Revert "Add completion styling"

This reverts commit eca1f90cf47a2edc1a1cd22e12c6c0f3b900654e.  Several
changes were requested, and it seemed simplest to revert it.

gdb/ChangeLog
2020-05-23  Tom Tromey  <tom@tromey.com>

	Revert commit eca1f90c:
	* NEWS: Remove entry for completion styling.
	* completer.c (_rl_completion_prefix_display_length): Move
	declaration later.
	(gdb_fnprint): Revert.
	(gdb_display_match_list_1): Likewise.
	* cli/cli-style.c (completion_prefix_style)
	(completion_difference_style, completion_suffix_style): Remove.
	(_initialize_cli_style): Revert.
	* cli/cli-style.h (completion_prefix_style)
	(completion_difference_style, completion_suffix_style): Don't
	declare.

gdb/doc/ChangeLog
2020-05-23  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Output Styling): Don't mention completion styling.
	(Editing): Don't mention readline completion styling.

gdb/testsuite/ChangeLog
2020-05-23  Tom Tromey  <tom@tromey.com>

	* gdb.base/style.exp: Remove completion styling test.
	* lib/gdb-utils.exp (style): Remove completion styles.