Do not set up ccache in CI jobs building docs

Building documentation for Automake-based BIND 9 versions does not
require running "make all" beforehand.  Drop ccache setup code from the
definitions of GitLab CI jobs building documentation as it is redundant.

(cherry picked from commit 59c8321ac9)

.gitlab-ci.yml

@@ -531,8 +531,6 @@ docs:
   <<: *release_branch_triggering_rules
   <<: *base_image
   <<: *docs_job
-  before_script:
-    - test -w "${CCACHE_DIR}" && export PATH="/usr/lib/ccache:${PATH}"
   needs:
     - job: autoreconf
       artifacts: true
@@ -544,7 +542,6 @@ docs:tarball:
   <<: *base_image
   <<: *docs_job
   before_script:
-    - test -w "${CCACHE_DIR}" && export PATH="/usr/lib/ccache:${PATH}"
     - *unpack_release_tarball
   needs:
     - job: tarball-create