test-qga: Kill broken and dead QGA_TEST_SIDE_EFFECTING code

Back when the test was introduced, in commit 62c39b307, the
test was set up to run qemu-ga directly on the host performing
the test, and defaults to limiting itself to safe commands.  At
the time, it was envisioned that setting QGA_TEST_SIDE_EFFECTING
in the environment could cover a few more commands, while noting
the potential danger of those side effects running in the host.

But this has NEVER been tested: if you enable the environment
variable, the test WILL fail.  One obvious reason: if you are not
running as root, you'll probably get a permission failure when
trying to freeze the file systems, or when changing system time.
Less obvious: if you run the test as root (wow, you're brave), you
could end up hanging if the test tries to log things to a
temporarily frozen filesystem.  But the cutest reason of all: if
you get past the above hurdles, the test uses invalid JSON in
test_qga_fstrim() (missing '' around the dictionary key 'minimum'),
and will thus fail an assertion in qmp_fd().

Rather than leave this untested time-bomb in place, rip it out.
Hopefully, as originally envisioned, we can find an opportunity
to test an actual sandboxed guest where the guest-agent has
full permissions and will not unduly affect the host running
the test - if so, 'git revert' can be used if desired, for
salvaging any useful parts of this attempt.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
