Inline expression constructor

Inline expression constructor

The struct expression constructor no longer does any real work, so
this inlines it.  The default destructor can also be used now as well.

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* parse.c (expression::expression, expression::~expression):
	Remove.
	* expression.h (struct expression): Inline constructor.  Remove
	destructor.