Use htab_up in dwarf2/read.c

Use htab_up in dwarf2/read.c

This changes dwarf2/read.c to use htab_up rather than explicit calls
to htab_delete.

gdb/ChangeLog
2020-09-17  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (compute_compunit_symtab_includes): Use htab_up.