t7004-tag: delete unnecessary tags with test_when_finished

t7004-tag: delete unnecessary tags with test_when_finished

The '--force is moot with a non-existing tag name' test creates two
new tags, which are then deleted right after the test is finished,
outside the test_expect_success block, allowing 'git tag -d's output to
pollute the test output.

Use test_when_finished to delete those tags.

Signed-off-by: SZEDER Gábor <redacted>
Signed-off-by: Junio C Hamano <redacted>