NR: remove unused unit: Cave Serpent

Part of #8734
Note: I searched all NR scenario files with "search and replace" in VS Code and apparently this unit was not even being used in NR.

data/campaigns/Northern_Rebirth/_main.cfg

@@ -78,10 +78,6 @@
     path=data/campaigns/Northern_Rebirth
 [/binary_path]
 
-[units]
-    {campaigns/Northern_Rebirth/units}
-[/units]
-
 [lua]
     code="wesnoth.require 'campaigns/Northern_Rebirth/lua/respawn_utils.lua'"
 [/lua]

data/campaigns/Northern_Rebirth/units/Cave_Serpent.cfg

@@ -1,14 +0,0 @@
-#textdomain wesnoth-nr
-[unit_type]
-    id=Cave Serpent
-    [base_unit]
-        id=Water Serpent
-    [/base_unit]
-    description=_"The Water Serpent is a foe dire enough when encountered in the rivers and seas of the sunlit world, but his rare cave-dwelling kin are yet more terrible. Though their eyes are of little use in the stygian darkness of the deep caves where they dwell, they have become sensitive to even the tiniest sound that might indicate unwary prey in their domain, and will home in on it with remorseless efficiency."
-    # We used to deploy a Sea Serpent here. The poison ability from
-    # the Water Serpent base type should do a pretty good job of
-    # compensating for the fact that the main attack is 6 points
-    # weaker than Sea Serpent's. We'll boost his HP a little to take
-    # uup the rest of the slack.
-    hitpoints=65
-[/unit_type]