jtag/core: remove unused variable

jtag/core: remove unused variable

Commit e3f3f60a02ab ("adapter speed: require init script setting
and centralize activation from drivers to core.c") has already
dropped the only use of variable 'jtag_speed'.

Remove the variable.

Change-Id: Iff096df0022982cf90795aa62d6b3406203f7b14
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6638
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>