Drop copy-pasted set_continue, dispatch_line

These were copy-pasted in from CPython in
https://github.com/inducer/pudb/commit/6065d5a2c53e953e00bf09f826a7cae3b1cadf13
According to https://bugs.python.org/issue16482, where the patch
originated, all supported versions of Python now have the fix.
Additionally, Python 3.13 has further modifications, which we
probably shouldn't copy.