python: Remove unused readline import
python: Remove unused readline import

The readline module is imported by main.py but doesn't seem to be
used.

Change-Id: I7888e3b6ad0a0fedc14b0feec2adf0a39883bbf8
Signed-off-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3421
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>

gem5/gem5,powerjg/gem5-ci-test,powerjg/gem5-ci-test,gem5/gem5,rallylee/gem5,TUD-OS/gem5-dtu,rallylee/gem5,TUD-OS/gem5-dtu,rallylee/gem5,rallylee/gem5,rallylee/gem5,TUD-OS/gem5-dtu,gem5/gem5,gedare/gem5,powerjg/gem5-ci-test,TUD-OS/gem5-dtu,sobercoder/gem5,gedare/gem5,gem5/gem5,powerjg/gem5-ci-test,gem5/gem5,gedare/gem5,gem5/gem5,powerjg/gem5-ci-test,sobercoder/gem5,gedare/gem5,gedare/gem5,sobercoder/gem5,sobercoder/gem5,gedare/gem5,gedare/gem5,sobercoder/gem5,TUD-OS/gem5-dtu,powerjg/gem5-ci-test,sobercoder/gem5,TUD-OS/gem5-dtu,powerjg/gem5-ci-test,sobercoder/gem5,gem5/gem5,rallylee/gem5,TUD-OS/gem5-dtu,rallylee/gem5