cmd/trace: remove unused variable in tests

cmd/trace: remove unused variable in tests

Unused variables in closures are currently not diagnosed by the
compiler (this is Issue #3059), while go/types catches them.

One unused variable in the cmd/trace tests is causing the go/types
test that typechecks the whole standard library to fail:

  FAIL: TestStdlib (8.05s)
    stdlib_test.go:223: cmd/trace/annotations_test.go:241:6: gcTime
    declared but not used
  FAIL

Remove it.

Updates #24464

Change-Id: I0f1b9db6ae1f0130616ee649bdbfdc91e38d2184
Reviewed-on: https://go-review.googlesource.com/101815
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>