Remove dwarf2_per_bfd::m_num_psymtabs

Remove dwarf2_per_bfd::m_num_psymtabs

Now that CUs and TUs are stored together in all_comp_units, the
m_num_psymtabs member is no longer needed -- it is always identical to
the length of the vector.  This patch removes it.

2021-05-30  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (struct dwarf2_per_bfd) <num_psymtabs,
	m_num_psymtabs>: Remove.
	(resize_symtabs): Update.
	* dwarf2/read.c (dwarf2_per_bfd::allocate_per_cu)
	(dwarf2_per_bfd::allocate_signatured_type): Update.