Can't delete the main Mac hard drive
If your volume is 'formatted' with coreStorage as a logical partition, reverting might be enough to resurrect the "Erase" tab.
In Terminal run the following command:

diskutil cs list


Note the disk identifier for the logical volume (e.g. on a Mac with only one drive and nothing else attached, it would be something like disk1)

Now try and revert from coreStorage (replacing disk1 with your own found disk identifier):

diskutil cs revert disk1


(you can run this either booted from your usual volume, or the installer disk)

If successful it will be mentioned in Terminal and the "Erase" button should have returned in Disk Utility, when you're booted from your installer drive.
(With more drives connected, make sure the disk identifier is the right one for the logical volume you want to revert)

If you can't successfully revert, make sure File Vault and encryption have been disabled, then try by destroying the whole coreStorage group with this command:

diskutil cs delete lvgUUID


replace lvgUUID with the UUID you found earlier in the diskutil cs list command under "Logical Volume Group".