stlink: remove 18 MHz jtag freq for stlink v2

stlink: remove 18 MHz jtag freq for stlink v2

While stlink v2 allows setting the jtag clock frequency till a max
of 18 MHz, the firmware seams unstable and not properly working.

Remove the entry for 18 MHz, at least until a fix get available.

Change-Id: I503e1b6a5709b5fbf1f1147fd3b5f34a0c5ee98c
Signed-off-by: Laurent LEMELE <laurent.lemele@st.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5465
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>