Remove CMAKE_USE_RELATIVE_PATHS variable.

The test for this variable was removed in commit v2.8.8~330^2~7 (complex:
Remove ancient unused ComplexRelativePaths test, 2011-12-23).

Commit v3.1.0-rc1~425^2~2 (backtrace: Convert to local paths in
IssueMessage, 2014-03-12) appears to have accidentally made some backtraces
print relative paths with the variable because conversions which used to be
done at configure time, before the variable had an effect are now potentially
done at generate time.

The documentation of the variable says not to use it, and the docs are wrong in
that the variable actually applies in per-directory scope.

The read of the variable makes it harder to split conversion methods from
cmLocalGenerator where they don't belong.  Remove it now.

Help/release/dev/remove-CMAKE_USE_RELATIVE_PATHS.rst

@@ -0,0 +1,5 @@
+remove-CMAKE_USE_RELATIVE_PATHS
+-------------------------------
+
+* The :variable:`CMAKE_USE_RELATIVE_PATHS` variable no longer has any
+  effect.  Previously it was partially implemented and unreliable.