Stop requiring vcvarsall.bat before build_mozc.py

With this commit 'vcvarsall.bat' (and its variant) are no longer
requires to be executed before running 'build_mozc.py', like
'build_qt.py' has never required 'vcvarsall.bat' (and its variant) [1].

'--vcvarsall_path' option, which is available in 'build_qt.py' [2],
is also available to explicitly specify a custom path of vcvarsall.bat
as follows.

  build_mozc.py --vcvarsall_path=C:\VS\VC\Auxiliary\Build\vcvarsall.bat

Overall this commit should make our build instructions simpler and
easier to keep maintaining.

Closes #923.

#codehealth

 [1]: baf418832c
 [2]: c777896808

PiperOrigin-RevId: 629293616

.github/workflows/windows.yaml

@@ -59,14 +59,12 @@ jobs:
         shell: cmd
         working-directory: .\src
         run: |
-          call "C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Auxiliary\Build\vcvarsamd64_x86.bat"
           python build_mozc.py gyp
 
       - name: build package
         shell: cmd
         working-directory: .\src
         run: |
-          call "C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Auxiliary\Build\vcvarsamd64_x86.bat"
           python build_mozc.py build -c Release package
 
       - name: upload Mozc64.msi
@@ -110,14 +108,12 @@ jobs:
         shell: cmd
         working-directory: .\src
         run: |
-          call "C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Auxiliary\Build\vcvarsamd64_x86.bat"
           python build_mozc.py gyp --noqt --msvs_version=2022
 
       - name: runtests
         shell: cmd
         working-directory: .\src
         run: |
-          call "C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Auxiliary\Build\vcvarsamd64_x86.bat"
           python build_mozc.py runtests -c Debug
 
   # actions/cache works without this job, but having this would increase the likelihood of cache hit