build: Removed unnecessary XML Documentation file directive from VC8 to VC12

build: Removed unnecessary XML Documentation file directive from VC8 to VC12

The curl tool project files for VC8 to VC12 would set this setting to
$(IntDir) which is the Visual Studio default value. To avoid confusion
when viewing settings from within Visual Studio and for consistency
with the libcurl project files removed this setting.

Conflicts:
	projects/Windows/VC10/src/curlsrc.tmpl
	projects/Windows/VC11/src/curlsrc.tmpl
	projects/Windows/VC12/src/curlsrc.tmpl
	projects/Windows/VC8/src/curlsrc.tmpl
	projects/Windows/VC9/src/curlsrc.tmpl