byacc: Disable backtracking extension.

When updating byacc to 20230201, I enabled the backtracking extension as it seemed useful and harmless. With a little more experience, it appears to have more impact than I initially thought and I'm no longer sure how useful it really is, so turn it back off.

PR: 244149, 269425
Sponsored by: Klara, Inc.
Reviewed by: imp, jkim
Differential Revision: https://reviews.freebsd.org/D38931