Removed ext_pillar backends are not synced

Description of Issue

If you remove a git repo from ext_pillars, the pillars are still there / in cache.

Setup

use ext_pillar git like e.g.

ext_pillar:
    - git:
        - master git@gitlab.my.domain:my/project.git
        - master git@gitlab.my.domain:my/other_project.git

Steps to Reproduce Issue

• remove my/other_project.git
• do a full sync
  • salt-run fileserver.update
  • salt-run git_pillar.update
  • salt-run saltutil.sync_all
  • salt-run cache.clear_all
  • salt '*' saltutil.refresh_pillar
  • salt '*' saltutil.sync_all
• do salt 'myminion' pillar.items
• old pillars are still there

Versions Report

Salt Version:
           Salt: 2019.2.0
 
Dependency Versions:
           cffi: Not Installed
       cherrypy: unknown
       dateutil: 2.6.1
      docker-py: Not Installed
          gitdb: 2.0.3
      gitpython: 2.1.8
          ioflo: Not Installed
         Jinja2: 2.10
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.5.6
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.15+ (default, Oct  7 2019, 17:39:04)
   python-gnupg: 0.4.1
         PyYAML: 3.12
          PyZMQ: 16.0.2
           RAET: Not Installed
          smmap: 2.0.3
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.2.5
 
System Versions:
           dist: Ubuntu 18.04 bionic
         locale: UTF-8
        machine: x86_64
        release: 4.4.0-134-generic
         system: Linux
        version: Ubuntu 18.04 bionic

Start by reproducing the listed ext_pillar git setup and the full sync sequence, including fileserver.update, git_pillar.update, cache.clear_all, and saltutil.refresh_pillar. Trace how removed git backends and their cached pillars are handled; done when removing a repository prevents its old pillars from appearing in pillar.items after the full sequence.