Remove premake sources

After 486509d5b9 we no longer use the
bundled sources, remove them.

Also remove the build.sh for building the bundled sources and update
.gitignore.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

.gitignore

@@ -29,24 +29,7 @@ libraries/source/spirv-reflect/*
 archives/
 source/tools/spirv/rules.*.json
 
-# premake5 build files, adapted from upstream
-build/premake/premake5/**/.gitignore
-build/premake/premake5/**/.travis.yml
-build/premake/premake5/.editorconfig
-build/premake/premake5/.github/
-build/premake/premake5/Bootstrap.*
-build/premake/premake5/CONTRIBUTING.md
-build/premake/premake5/FUNDING.yml
-build/premake/premake5/appveyor.yml
-build/premake/premake5/bin/release/premake5
-build/premake/premake5/build/*/obj/
-build/premake/premake5/build/codelite*
-build/premake/premake5/build/gmake2.windows/
-build/premake/premake5/build/vs200*/
-build/premake/premake5/build/vs201[0-5]/
-build/premake/premake5/build/xcode4/
-build/premake/premake5/packages/
-build/premake/.already-built
+# Files generated by running premake5
 build/premake/.gccmachine.tmp
 build/premake/.gccver.tmp