Correct Vampire Lurker Strain Damage Modifications (#5753)

# About the pull request

This PR corrects an erroneous change in `apply_strain` because only
modifiers can be set here. The change to lower the damage tier would be
undone when `recalculate_damage` would apply the lurker caste's values
for the upper and lower damage values.

Ultimately I'm not certain it does need the damage reduction - but as of
#5625 it does more with direct tail stabs so between the two this should
be a minor nerf.

# Explain why it's good for the game

Corrects an intended reduction in damage for taking the strain.

# Testing Photographs and Procedure
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<summary>Screenshots & Videos</summary>

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# Changelog
:cl: Drathek
balance: Corrected vampire lurker damage values (Effectively reduction
by 5 damage for most attacks)
/:cl:

code/modules/mob/living/carbon/xenomorph/strains/castes/lurker/vampire.dm

@@ -21,8 +21,7 @@
 	lurker.health_modifier -= XENO_HEALTH_MOD_MED
 	lurker.speed_modifier += XENO_SPEED_FASTMOD_TIER_1
 	lurker.armor_modifier += XENO_ARMOR_MOD_LARGE
-	lurker.melee_damage_lower = XENO_DAMAGE_TIER_3
-	lurker.melee_damage_upper = XENO_DAMAGE_TIER_3
+	lurker.damage_modifier -= XENO_DAMAGE_MOD_VERY_SMALL
 	lurker.attack_speed_modifier -= 2
 
 	lurker.recalculate_everything()