Removing items from CMakeFile

[demonpig]

Reason for removing check for webkit2gtk was that webkit2gtk-3.0
is no longer supported by Notes-Up.

Reason for removing PKG_DEPS variable is that it was unnecessary
since package dependencies are already checked before the variable
is set.

[demonpig]

This commit will allow people with Cmake 3.8 to compile the program without editing the CMakeFile.

Reference: #199

[Mailaender]

I can confirm that this patch helps https://build.opensuse.org/request/show/512129

[Philip-Scott]

@demonpig You've officially killed Notes-Up updates for people that are still in Freya or bellow 😄

Yeah, the reason that was still there is because it's being still built for Freya. But, it's a good day to make people update, don't you think 😉

[demonpig]

@Philip-Scott Whoops! :)

I do wonder how the build was working anyways since I tried to build the application on Freya but received build errors due to webkit2gtk-4 being required. Regardless, i am working on the AppImage of the application so if they need an updated version, then people can use that.