Drop capabilities stealth and half-stealth

These were leftovers from Chainsaw, and their only remaining effect
was a flak bonus.

The got replaced except for flak by plane selector stealth under
Chainsaw option STEALTHV, which became mandatory in Empire 2.  It is
unclear why STEALTHV didn't cover flak.  No planes with these
capabilities have existed in the stock game since Empire 2.

src/lib/global/symbol.c

@@ -208,9 +208,7 @@ struct symbol plane_chr_flags[] = {
     {P_S, "spy"},
     {P_I, "image"},
     {P_O, "satellite"},
-    {P_X, "stealth"},
     {P_N, "SDI"},
-    {P_H, "half-stealth"},
     {P_E, "x-light"},
     {P_K, "helo"},
     {P_A, "ASW"},