glsl: Remove ir_call::get_callee() and set_callee().

glsl: Remove ir_call::get_callee() and set_callee().

Previously, set_callee() performed some assertions about the type of the
ir_call; protecting the bare pointer ensured these checks would be run.

However, ir_call no longer has a type, so the getter and setter methods
don't actually do anything useful.  Remove them in favor of accessing
callee directly, as is done with most other fields in our IR.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>