Remove leftover tests.rest.spec property from docs (#29279)

We previously had a property to specify the location of the REST test
spec files but this was removed in a previous refactoring yet left
behind in the docs. This commit removes the last remaining vestige of
this parameter.

TESTING.asciidoc

@@ -296,7 +296,6 @@ e.g. -Dtests.rest.suite=index,get,create/10_with_id
 * `tests.rest.blacklist`: comma separated globs that identify tests that are
 blacklisted and need to be skipped
 e.g. -Dtests.rest.blacklist=index/*/Index document,get/10_basic/*
-* `tests.rest.spec`: REST spec path (default /rest-api-spec/api)
 
 Note that the REST tests, like all the integration tests, can be run against an external
 cluster by specifying the `tests.cluster` property, which if present needs to contain a