Remove duplicate (likely incorrect) morse definition for '-'

The Morse table contains two conflicting definitions for '-':

1. "-...-" on L101 (henceforth "3dit")
2. "-....-" on L108 ("4dit")

The order of the list meant that 3dit was used as it came first.
However it also used 3dit for `=` (L107).

Multiple sources, most notably the International Telecommunication Union
Recommendation for Morse code (ITU-R;
https://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.1677-1-200910-I!!PDF-E.pdf),
use 4dit for `-`, and I found nobody else using 3dit.

As such, especially given the conflicting definitions, I think 3dit is
an error, and this commit removes it.

src/morse.c

@@ -98,7 +98,6 @@ static const struct morse_s {
 	{ '8', "---.." },
 	{ '9', "----." },
 	{ '0', "-----" },
-	{ '-', "-...-" },
 	{ '.', ".-.-.-" },
 	{ ',', "--..--" },
 	{ '?', "..--.." },