I was finally able to clean out the garbage. The remnants from EndeavourOS were located on the uefi-boot-esp-fat32-partition. Here's what I did:
1) Boot to Solus-live-usb (I don't know if this is necessary)
2) $ sudo mount -o rw /dev/nvme0n1p1 /boot
- The name of the partition "nvme0n1p1" I found out using GParted
3) Navigate to /boot directory using the nemo filemanager.
4) One directory named 59415ac2eed841f7a1920447b912c084 seemed to be the related to EndeavourOS as it contained subdirectories like 6.1.24-1-lts and 6.2.11-arch1-1. I deleted this directory.
5) $ sudo umount /dev/nvme0n1p1 /boot
Now the solus uefi-boot-menu is nice and clean.
I wonder why my LinuxMint boot entry is not listed though?