Remove obsolete Gitpod files

In PR #11300 Gitpod support got introduced, but we re-evaluated that
decision in #11732. In PR #11800 the support was partially reverted,
but the actual Gitpod files were kept to not outright break potential
workflows because at the time we were not sure if, and if so how often,
Gitpod was actually used for contributing to PDF.js.

However, in addition to the concerns mentioned in #11732 after five
years we haven't seen any contributions that clearly originated from
Gitpod, and the files have not been updated after e.g. PR #11807 and
PR #17913 because it's not a workflow that we maintain or are able
to test (nor have we seen Gitpod community contributions for this).

This commit therefore removes the remaining Gitpod files to reduce
maintainance burden for PDF.js. Note that users of Gitpod can still
contribute to PDF.js via the platform; we just don't provide/manage
workspace files from this repository anymore.

.gitpod.Dockerfile

@@ -1,7 +0,0 @@
-FROM gitpod/workspace-full-vnc
-
-USER gitpod
-
-RUN sudo apt-get update && \
-    sudo apt-get install -yq firefox && \
-    sudo rm -rf /var/lib/apt/lists/*

.gitpod.yml

@@ -1,13 +0,0 @@
-image:
-  file: .gitpod.Dockerfile
-tasks:
-  - command: |
-      gp await-port 8888 && gp preview $(gp url 8888)/web/viewer.html && echo '[{"name": "Firefox","path": "/usr/bin/firefox"}]' | jq '.' > test/resources/browser_manifests/browser_manifest.json
-
-  - init: npm install -g gulp-cli && npm install
-    command: gulp server
-ports:
-  - port: 8888
-    onOpen: ignore
-  - port: 6080
-    onOpen: ignore