server: Revert "Report backtraces in codemodel response"
The backtrace information is very repetitive and hugely increases the size of the codemodel object. We need to remove it until an alternative representation can be developed. Revert !992 (merged), except for the protocol version number (because it indicates other new things).
Unfortunately this is incompatible with clients that expect the "crossReferences" field in targets. However, the regression in memory usage is quite serious, especially on large projects, and therefore breaks even older clients that do not use backtraces. Since the "crossReferences" field was only provided by one release (3.10.0), it is simplest to revert it outright for 3.10.1.
Backtrace information was included in test info by !1414 (merged) to match that already provided for targets. Drop that too for now.
Fixes: #17502 (closed)