[doc] remove bullet point referring to Node.js 12

Refs: #43734 (review)

Author: lpinca

doc/contributing/maintaining-openssl.md

@@ -11,11 +11,6 @@ currently need to generate four PRs as follows:
   of this guide.
 * a PR for 14.x following the instructions in the v14.x-staging version
   of this guide.
-* a PR which uses the same commit from the third PR to apply the
-  updates to the openssl source code, with a new commit generated
-  by following steps 2 onwards on the 12.x line. This is
-  necessary because the configuration files have embedded timestamps
-  which lead to merge conflicts if cherry-picked from the second PR.
 
 ## Use of the quictls/openssl fork