trace2_event: Add remove_prefix to fix failing tests on Linux & macOS.

removeprefix is available i python 3.9. Mac and Linux are running in
a version below 3.9. Thus tests are failing with the following error:
  "AttributeError: 'str' object has no attribute 'removeprefix' "

Replaced the removeprefix with custom function which we will delete
once Linux and macOS versions are updated.

Tested:
$ ./run_tests

Bug: [google internal] b/201453085
Change-Id: I9b4d564ff1176e1b4471805ef05472c1914cd9f9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/319375
Tested-by: Raman Tenneti <rtenneti@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

0 files changed, 0 insertions, 0 deletions