Remove all Git-related functionality

LOOT no longer uses Git to keep its copies of the masterlists up to
date, so this functionality is no longer needed. The removed API items
are:

- UpdateFile()
- GetFileRevision()
- IsLatestFile()
- libgit2_category()
- GitStateError

.github/workflows/ci.yml

@@ -118,8 +118,6 @@ jobs:
       - name: Run tests
         run: |
           cd build
-          git config --global user.email "github@github-actions"
-          git config --global user.name "GitHub"
           ./libloot_internals_tests
           ./libloot_tests
 
@@ -200,8 +198,6 @@ jobs:
       - name: Run tests
         run: |
           cd build/${{ env.MSVC_CONFIG }}
-          git config --global user.email "github@github-actions"
-          git config --global user.name "GitHub"
           .\libloot_internals_tests.exe
           .\libloot_tests.exe
 

.github/workflows/release.yml

@@ -94,8 +94,6 @@ jobs:
       - name: Run tests
         run: |
           cd build
-          git config --global user.email "github@github-actions"
-          git config --global user.name "GitHub"
           ./libloot_internals_tests
           ./libloot_tests