Remove impossible version export cruft

In Monero 12.0 the version code got rewritten and, as a result, the
variable substitution export attribute no longer applies (since
`version.cmake` got moved and renamed to `Version.cmake`), but this bit
of cruft to rewrite the release tag to "-release" and the attribute are
still there, but cannot ever be invoked.

Remove them.  I specifically don't want to fix it because source exports
can come from anywhere, *not* only tagged versions, so putting
"-release" in the version string could be highly misleading.

.gitattributes

@@ -1,2 +1 @@
 .git* export-ignore
-version.cmake export-subst

cmake/Version.cmake

@@ -32,10 +32,7 @@ function (write_static_version_header hash)
 endfunction ()
 
 find_package(Git QUIET)
-if ("$Format:$" STREQUAL "")
-  # We're in a tarball; use hard-coded variables.
-  write_static_version_header("release")
-elseif (GIT_FOUND OR Git_FOUND)
+if (GIT_FOUND OR Git_FOUND)
   message(STATUS "Found Git: ${GIT_EXECUTABLE}")
   add_custom_command(
     OUTPUT            "${CMAKE_BINARY_DIR}/version.cpp"