Remove getOverlayMount() and getGlobalDebugMount()

[felixfbecker]

Fixes #3228.

Only GitHub used this, but shouldn't, see #3228.
After removing, they always append to document.body, so there is not reason to have them configurable or subscribe to the mutations (which has a perf impact). It's safe to assume document.body never gets removed from any code host DOM.

Test Plan

Code Hosts

• GitHub
• GitHub Enterprise
• Refined GitHub
• Phabricator
• Phabricator integration
• Bitbucket
• Gitlab

Browsers

• Chrome
• Firefox

[codecov]

Codecov Report

Merging #3318 into master will decrease coverage by 0.11%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
client/browser/src/libs/github/extensions.tsx	94.11% <ø> (-2.18%)	⬇️
.../code_intelligence/code_intelligence_test_utils.ts	92.3% <100%> (+0.3%)	⬆️
...lient/browser/src/libs/github/code_intelligence.ts	41.5% <100%> (-4.58%)	⬇️
...r/src/libs/code_intelligence/code_intelligence.tsx	64.07% <100%> (-0.05%)	⬇️
cmd/frontend/backend/go_importers.go	22.22% <0%> (-37.5%)	⬇️
cmd/frontend/backend/repos_vcs.go	22.03% <0%> (-7.63%)	⬇️
cmd/frontend/backend/repos.go	28.71% <0%> (-2.98%)	⬇️

[lguychard]

👍