openocd: remove further obsolete hooks for building on eCos

The eCos build has been dropped in 2012 with commit 39650e2273
("ecosboard: delete bit-rotted eCos code").

Drop last eCos related code.

Change-Id: I6b0de6386386c0102d540504236dda58d7a664b9
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/9092
Tested-by: jenkins

doc/manual/server.txt

@@ -190,7 +190,6 @@ Remember: OpenOCD runs on:
 -# FreeBSD
 -# Cygwin
 -# MinGW32
--# Ecos
 
 How can we get that to work?