Macros: fix leaked variable in {HEAL_UNIT}

[heal_unit] set's as side-effect an variable, which contains the heal amount
of one of the healed units.
It's more likely that this will just add a variable with authors not being aware
to clean it afterwards, than the case being that someone want's this behaviour.

I assume that this happens is an unintended side-effect from former code refactoring,
the original implementation of this macro was not doing this.

[ci skip]

changelog.md

@@ -56,6 +56,7 @@
      measured in milliseconds.
    * Added unit type variation selection to the debug mode Create Unit dialog (PR #4986).
    * Removed the Python2 versions of tools.
+   * {HEAL_UNIT} does no longer set the variable heal_amount. If needed, [heal_unit] can be used instead.
 
 ## Version 1.15.3
  ### AI

data/core/macros/utils.cfg

@@ -350,6 +350,9 @@
         amount=full
         restore_statuses=yes
     [/heal_unit]
+    [clear_variable]
+        name=heal_amount
+    [/clear_variable]
 #enddef
 
 #define PUT_TO_RECALL_LIST FILTER