CLN: Removed all occurances of `goal_time` and enabled linting for benchmarks
- [X] closes #23075
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

The following changes were made:

- All occurances of `goal_time` have been removed. Running `grep -rnw asv_bench/benchmarks/ -e goal_time --include "*.py"` should no longer return anything.
- The benchmark folder is no longer excluded in `setup.cfg`
- Removed one unused import in `asv_bench/benchmarks/io/excel.py` to pass flake8.
