Remove NMake support (#10656)

* Deleting NMake build

* Deleting documentation for NMake

cmake/README.md

@@ -89,9 +89,6 @@ for various native build systems.
 
 Of most interest to Windows programmers are the following:
 
-* [Makefile](http://www.cmake.org/cmake/help/latest/manual/cmake-generators.7.html#makefile-generators).
-  This generates NMake Makefiles for Visual Studio. These work, but they are rather slow.
-
 * [Visual Studio](http://www.cmake.org/cmake/help/latest/manual/cmake-generators.7.html#visual-studio-generators)
   This generates a Visual Studio solution for the project.
 
@@ -112,16 +109,6 @@ Create a temporary *build* folder and change your working directory to it:
 The *Makefile* and *Ninja* generators can build the project in only one configuration, so you need to build
 a separate folder for each configuration.
 
-To start using a *Release* configuration via the *NMmake* generator:
-
-     C:\Path\to\build\protobuf>mkdir release & cd release
-     C:\Path\to\build\protobuf\release>cmake -G "NMake Makefiles" ^
-     -DCMAKE_BUILD_TYPE=Release ^
-     -DCMAKE_INSTALL_PREFIX=C:\Path\to\install ^
-     C:\Path\to\src\protobuf
-
-It will generate a *NMake* *Makefile* in the current directory.
-
 To use *Debug* configuration using *Ninja*:
 
      C:\Path\to\build\protobuf>mkdir debug & cd debug
@@ -192,10 +179,6 @@ Note that if your generator supports multiple configurations, you will probably
 
 You can also run directly the build tool you've configured:
 
-     C:\Path\to\build\protobuf\release>nmake
-
-or
-
      C:\Path\to\build\protobuf\debug>ninja
 
 And wait for the compilation to finish.
@@ -268,10 +251,6 @@ To install protobuf to the *install* folder you've specified in the configuratio
 
 Or if you prefer:
 
-     C:\Path\to\build\protobuf\release>nmake install
-
-or
-
      C:\Path\to\build\protobuf\debug>ninja install
 
 You can also build project *INSTALL* from Visual Studio solution.