server: drop "ctestInfo" backtrace information

Backtrace information was included by commit 35a52bd1b4 (server: add
"ctestInfo" request to get test info, 2017-10-25) to match that already
provided for targets.  However, the backtrace representation uses too
much memory and needs to be dropped.  Remove it from test information.

Issue: #17502

Help/manual/cmake-server.7.rst

@@ -669,17 +669,6 @@ Each test object can have the following keys:
   contains the test command.
 "properties"
   contains a list of test property objects.
-"backtrace"
-  contains a list of backtrace objects that specify where the test was defined.
-
-Each backtrace object can have the following keys:
-
-"path"
-  contains the full path to the file containing the statement.
-"line"
-  contains the line number in the file where the statement was defined.
-"name"
-  contains the name of the statement that added the test.
 
 Each test property object can have the following keys:
 

Source/cmServerProtocol.cxx

@@ -799,9 +799,6 @@ static Json::Value DumpCTestInfo(cmTest* testInfo)
   }
   result[kPROPERTIES_KEY] = properties;
 
-  // Need backtrace to figure out where this test was originally added
-  result[kBACKTRACE_KEY] = DumpBacktrace(testInfo->GetBacktrace());
-
   return result;
 }