Remove VC14 (VS2015/VS2017 project files).

Reasons:
* These projectfiles are still 32-bit, whereas all other builds for all other OSes are now 64-bit.
* Relatedly, these projectfiles are dependent on the libraries kept at the aquileia/external repository.
* VS2019 is still listed as supporting Windows 7, so there is a minimal likelihood of developers not being able to use VS2019.
* The VS2019 projectfiles instead use vcpkg to get the required dependencies, which is easier to setup than the aquileia/external prebuilt libraries.
* It's one less thing that needs to be updated whenever source files are added/moved/removed.
* It's two fewer jobs that Travis needs to run, which means Travis builds will finish more quickly.

.gitignore

@@ -50,7 +50,6 @@ projectfiles/**/*objs*
 
 # Visual Studio
 projectfiles/VC*
-!projectfiles/VC14
 !projectfiles/VC16
 projectfiles/VC*/.vs/
 projectfiles/VC*/*.vc.*db

.travis.yml

@@ -8,8 +8,6 @@ cache:
   directories:
     - $HOME/build-cache
     - $HOME/flatpak-cache
-    - $TRAVIS_BUILD_DIR/projectfiles/VC14/Debug
-    - $TRAVIS_BUILD_DIR/projectfiles/VC14/Release
     - $TRAVIS_BUILD_DIR/projectfiles/VC16/Debug
     - $TRAVIS_BUILD_DIR/projectfiles/VC16/Release
     - $HOME/vcpkg
@@ -23,12 +21,6 @@ env:
 
 jobs:
   include:
-    - os: windows
-      env: TOOL=msbuild NLS=false IMAGE=VC14 BRANCH=master CFG=Debug
-
-    - os: windows
-      env: TOOL=msbuild NLS=false IMAGE=VC14 BRANCH=master CFG=Release
-
     - os: windows
       env: TOOL=msbuild NLS=false IMAGE=VC16 BRANCH=master CFG=Debug UPLOAD=true
 

INSTALL.md

@@ -78,7 +78,7 @@ rebuild Wesnoth frequently (i.e. for development and testing).
 See [here](https://github.com/wesnoth/wesnoth/blob/master/projectfiles/Xcode/README.md) for instructions on using Xcode.
 
 ### Windows
-See [here](https://github.com/wesnoth/wesnoth/blob/master/projectfiles/VC14/README.md) for instructions on using Visual Studio 2015/2017, or [here](https://github.com/wesnoth/wesnoth/blob/master/projectfiles/VC16/README.md) for (much simpler) instructions on using Visual Studio 2019.
+See [here](https://github.com/wesnoth/wesnoth/blob/master/projectfiles/VC16/README.md) for instructions on using Visual Studio 2019.
 
 ## SCons Build