Partition my HD with Disk Utility, cannot resize the partition WITHOUT installing Windows. WTF?
Here are instructions for undoing a Core Storage conversion (source - this method worked for me). It should be non-destructive, but as always, take a backup first! 


Boot into Recovery HD. If for any reason you don't have Recovery HD, then you'll have to make a bootable Yosemite drive and boot into that.
In the Menu Bar, select Utilities > Terminal.
Run diskutil cs list. You'll get an output that looks like this (credits to the Ask Different user in my source):






Select your Logical Volume ID (the one highlighted in the screenshot) and copy it to the clipboard.
Run the Terminal command diskutil cs revert Paste-Your-Logical-Volume-ID-Here.
Run diskutil cs list to verify that your computer can't find any CS volumes.
You're done! Now Disk Utility should be able to do everything you need it to do to your HD!