Revert "MEDIUM: checks: capture groups in expect regexes"

Revert "MEDIUM: checks: capture groups in expect regexes"

This reverts commit 1979943c30ef285ed04f07ecf829514de971d9b2.

Captures in comment was only used when a tcp-check expect based on a negative
regex matching failed to eventually report what was captured while it was not
expected. It is a bit far-fetched to be useable IMHO. on-error and on-success
log-format strings are far more usable. For now there is few check sample
fetches (in fact only one...). But it could be really powerful to report info in
logs.