grunt: Remove pull-up on UART pins.
grunt: Remove pull-up on UART pins.

BUG=b:73135908
BRANCH=None
TEST=Flash grunt; verify EC UART still functional.  Reboot Cr50 a bunch
without servo connected, verify that Cr50 never reports servo as
"connected".

Change-Id: I1ec402b4e3d0e9debdbb2af3a1ba5e1c45aa655a
Signed-off-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@google.com>
Reviewed-on: https://chromium-review.googlesource.com/917182
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Tested-by: Edward Hill <db1406b34e091703092b6b136bdfeb2c8125fc0e@chromium.org>
Reviewed-by: Aaron Durbin <af0622ab041e8b0f8e5819838dcc93898196e71f@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec