[ios] removed conflicted config-file target to get me worked with device plugin.

To reproduce it,
1. create a project
2. add platform ios and plugin device & device-capability
3. check out config.xml at platforms/ios/project-name/config.xml, device feature was existed
4. run 'xmen prepare', check out config.xml again, device feature gone!

reason:
1. cli keeps refercence count by 'feature' tag
2. cli will re-create config.xml during 'prepare' stage, but if reference count of one feature is more than one (our case), it won't write that feature back to config.xml, that's why we lost 'device' feature finally

solution:
device-capability doesn't really need this config, it depends on device, it shares same feature name with device, that's fair enough