jtag: retire jtag_alloc_in_value32 API

not needed, reduce area of interface and sharp edges to
API.

Change-Id: I5347352e7595686634bd0de13fcf6de6e55027b0
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
Reviewed-on: http://openocd.zylin.com/262


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>