tests/avocado: remove tcg_plugins virt_mem_icount test

tests/avocado: remove tcg_plugins virt_mem_icount test

Since 4f8d886085 (tests/plugin/mem: migrate to new per_vcpu API) this
test was skipping due to not being able to run callback and inline
memory instrumentation at the same time.

However b480f7a621 (tests/plugin: add test plugin for inline
operations) tests for all this matching up so we don't need the
additional complexity in avocado.

Remove the test.

Fixes: 4f8d886085
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240729144414.830369-9-alex.bennee@linaro.org>