Remove /MP from common.gypi

Forcing this option makes the option ```MultiProcessorCompilation``` in ```msvs_settings``` useless.  It also means that one cannot use ```#import``` or they will be faced with this error when trying to build:

```
error C2813: #import is not supported with /MP (compiling source file...)
```

Please see additional discussion of this here:  nodejs/node-gyp#1087
and here: nodejs/node-gyp#26

common.gypi

@@ -150,7 +150,6 @@
             'BasicRuntimeChecks': 3, # /RTC1
             'AdditionalOptions': [
               '/bigobj', # prevent error C1128 in VS2015
-              '/MP', # compile across multiple CPUs
             ],
           },
           'VCLinkerTool': {