internal/filedesc: fix parsing of enforce_utf8 option
internal/filedesc: fix parsing of enforce_utf8 option

Remove spurious negation.

Change-Id: I484fa6fecda85943cdedd96a6c6f0f2349f6bfee
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/188338
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go