Remove some unneeded psymtab initializations

allocate_psymtab has long cleared the new psymtab that is returned.
This patch documents this behavior and then removes some redundant
initializations.

gdb/ChangeLog
2019-01-10  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (xcoff_end_psymtab): Remove some initializations.
	* psymtab.c (allocate_psymtab): Add comment.
	* psympriv.h (allocate_psymtab): Add comment.
	* dwarf2read.c (dwarf2_create_include_psymtab): Remove some
	initializations.
	* dbxread.c (dbx_end_psymtab): Remove some initializations.