bump-formula-pr: Only delete Linux bottle line if it exists

- We have some Linux-only formulae that don't have bottles.
- Previously, bumping a Linux-only formula that didn't have a bottle
  line - eg, `adoptopenjdk` which is `bottle :unneeded` - would fail:

```
Error: inreplace failed
/app/linuxbrew/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/adoptopenjdk.rb:

expected replacement of /^    sha256 ".+" => :x86_64_linux\n/m with "\2"
```