Removed lfs.extension.* from list of supported keys for .lfsconfig
Removed lfs.extension.* from list of supported keys for .lfsconfig

The LFS extension keys are not considered safe by Git LFS so attempting to put them in `.lfsconfig` results in such warnings:
```
WARNING: These unsafe lfsconfig keys were ignored:

  lfs.extension.gzip.clean
  lfs.extension.gzip.smudge
```
github/git-lfs,github/git-lfs,github/git-lfs