Clean up the iOS build script (#25822)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25822
### Summary
Since protobuf has been removed from mobile, the `build_host_protoc.sh` can be removed from `build_ios.sh` as well. However, the old caffe2 mobile build still depend on it, therefore, I introduced this `BUILD_PYTORCH_MOBILE` flag to gate the build.
- iOS device build
```
BUILD_PYTORCH_MOBILE=1 IOS_ARCH=arm64 ./scripts/build_ios.sh
BUILD_PYTORCH_MOBILE=1 IOS_ARCH=armv7s ./scripts/build_ios.sh
```
- iOS simulator build
```
BUILD_PYTORCH_MOBILE=1 IOS_PLATFORM=SIMULATOR ./scripts/build_ios.sh
```
### Test Plan
All device and simulator builds run successfully
Test Plan: Imported from OSS
Differential Revision: D17264469
Pulled By: xta0
fbshipit-source-id: f8994bbefec31b74044eaf01214ae6df797816c3
