Remove clear_section_table

The call to clear_section_table in ~program_space is now clearly not
needed -- the section table will clear itself.  This patch removes
this call and then inlines the one remaining call to
clear_section_table.

gdb/ChangeLog
2020-10-12  Tom Tromey  <tom@tromey.com>

	* progspace.c (program_space::~program_space): Don't call
	clear_section_table.
	* exec.h (clear_section_table): Don't declare.
	* exec.c (exec_target::close): Update.
	(clear_section_table): Remove.