chore(vrl): Revert immutable target for conditions (#12844)

Stumbled across this again while putting together the release notes for
0.22.0.

Upon reflecting on this more, I think this is liable to surprise users
that assignements are silently ignored in conditions. I think I'd prefer
to just defer this work until we can actually support it at the compiler
level to error if users try to assign in a condition given the gains
here don't seem to justify the risk,  though I'm curious what this PR
shows as the regression given the other work that has happened since it
merged.

Reverts: #11916

Signed-off-by: Jesse Szwedko <jesse@szwedko.me>