Linux – Windows/Linux dualboot: Windows asks for bitlocker recovery key every boot after reinstalling Linux

You need to log into Windows 7, suspend Bitlocker (no need to disable your TPM), then reboot your computer into Windows 7 again (and reenable Bitlocker if it doesn't automatically do it). This will "reset" Bitlocker to understand your new dual-boot configuration. Once you've done this, you should not need to enter your recovery key the next time you boot into Windows 7.

The optimal solution is to disable BitLocker completely, as you said. However, there is a way to keep Bitlocker on your computer and keep Ubuntu. I have not tried this on my own computer but it has worked well with others.

The way to do this is to uninstall Ubuntu. Remember to disable BitLocker from Windows. Provided that you have files you want to keep, I would suggest you take a backup (I would do this of the /home folder because that is where all your user files are (you would need to reinstall all user-installed programs and dependencies); otherwise you can just take a full backup and reinstall GRUB; more info about this is in this question.

The way to uninstall Ubuntu is easy. Boot from your LiveCD or create a USB with Rufus or some other utility and boot from that. After, open GParted and simply delete the partitions. However, you're not done. Because you deleted the partitions, you also deleted the grub files configuration files (everything in /boot/grub). As a result, if you boot up your computer now, you will get to a grub-rescue screen. So don't reboot just yet.

Open a terminal and run these commands to add a PPA and install boot-repair:

sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install boot-repair && boot-repair

After that, select "Recommended repair." This will create a new BCD for your Windows system and reinstall bootmgr to the MBR. It will ask you to create a pastebin after and give you a file telling you about what happened. After that, reboot. This should hopefully work. Otherwise, you can clone the drive to another one of the same size and format the whole drive, then move Windows back. Again, the best (and least risky) solution is to disable BitLocker completely.