Revert "Turn off threaded minsym demangling by default"

Revert "Turn off threaded minsym demangling by default"

This reverts commit 62e77f56f0ce8b10122881d8f0acd70e113fde93.
(except for ChangeLog and a bugfix in minimal_symbol_reader::install)

As agreed on the mailing list, now that GDB 9 has branched, this patch
reverts the change to set worker-threads to zero. After this patch,
multithreaded minsym demangling will be enabled again by default.

gdb/ChangeLog:

2019-12-13  Christian Biesinger  <cbiesinger@google.com>

	* maint.c (n_worker_threads): Default to -1.
	(worker_threads_disabled): Remove function.
	* maint.h (worker_threads_disabled): Remove function.
	* minsyms.c (minimal_symbol_reader::record_full): Don't call
	symbol_set_names here if worker_threads_disabled () is true.

Change-Id: I5ff3e318d96f60968c8b8bedb84546ad2314d94b