build: Removed unnecessary Precompiled Header file directive in VC7 to VC12

The curl tool project files for VC7 to VC12 would set this settings to
$(IntDir)$(TargetName).pch 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