Bug 1686967 - Don't export pdbstr from win64 mozconfig. r=firefox-build-system-reviewers,mhentges

Since bug 1686646, we don't set the variable for configure, we instead
set it for convenience to use it in two places.

Differential Revision: https://phabricator.services.mozilla.com/D102045

browser/config/mozconfigs/win64/common-win64

@@ -25,7 +25,7 @@ unset VC_PATH
 
 fi
 
-export pdbstr="${MOZ_FETCHES_DIR}/pdbstr/pdbstr.exe"
+pdbstr="${MOZ_FETCHES_DIR}/pdbstr/pdbstr.exe"
 if [ -f "${pdbstr}" ]; then
     chmod +x "${pdbstr}"
 fi