server: remove remaining crust from dropped eCos code

Commit 39650e2273 ("ecosboard: delete bit-rotted eCos code") has
removed eCos code but has left some empty function that was used
during non-eCos build to replace eCos mutex.

Drop the functions and the file that contain them.

Change-Id: I31bc0237ea699c11bd70921660f960ee406ffa80
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6835
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/server/Makefile.am

@@ -6,7 +6,6 @@ noinst_LTLIBRARIES += %D%/libserver.la
 	%D%/server.h \
 	%D%/telnet_server.h \
 	%D%/gdb_server.h \
-	%D%/server_stubs.c \
 	%D%/tcl_server.c \
 	%D%/tcl_server.h \
 	%D%/rtt_server.c \