Removing

You can remove your conditions by pressing the "Delete" button on the Conditional Formatting dialog box.
This will display the Delete dialog box.
Select which conditions you want to remove and press OK.

These changes are not made permanent until you press OK on the Conditional Formatting dialog box.

Removing Conditional Formatting

Pressing the Delete key to remove the contents from a cell will not remove the conditional formatting.
This is in the same way that pressing the Delete key does not remove the number format.
You press (Edit > Clear > Formats) to remove the conditional formatting.
A quick way to remove the conditional formatting is to copy a blank cell over the top.