arm/gd32f4: Remove extra lock from gd32f4xx_serial

[lupyuen]

Summary

An extra lock appears in gd32f4xx_serial due to 2 conflicting PRs:

• add chip GD32F450 of GD32MCU #7011
• Fixed errors in CI build tests for pull request #15351 #15352

This PR removes the extra lock in gd32f4xx_serial.

Impact

This PR will fix the gd32f4xx_serial errors reported by nuttx-dashboard.org

Testing

Before this PR: gd32f470zk-eval:nsh fails to build due to duplicate lock
https://github.com/NuttX/nuttx/actions/runs/12539724506/job/34966149597#step:7:998

Configuration/Tool: gd32f470zk-eval/nsh,CONFIG_ARM_TOOLCHAIN_GNU_EABI
Error: chip/gd32f4xx_serial.c:174:19: error: duplicate member 'lock'
  174 |   spinlock_t      lock;          /* Spinlock */
      |                   ^~~~

After this PR: gd32f470zk-eval:nsh builds successfully in Docker
https://gist.github.com/lupyuen/1e4095824e44561517eff9ebc028bfc8

sudo docker run \
  -it \
  ghcr.io/apache/nuttx/apache-nuttx-ci-linux:latest \
  /bin/bash
cd
git clone https://github.com/lupyuen2/wip-nuttx nuttx --branch gd32f4xx_serial
git clone https://github.com/apache/nuttx-apps apps
pushd nuttx ; echo NuttX Source: https://github.com/apache/nuttx/tree/$(git rev-parse HEAD) ; popd
pushd apps  ; echo NuttX Apps: https://github.com/apache/nuttx-apps/tree/$(git rev-parse HEAD) ; popd
cd nuttx
tools/configure.sh gd32f470zk-eval:nsh
make -j

NuttX Source: https://github.com/apache/nuttx/tree/d50cc1c518fb1e68e985df8489bffe4b4d71f22b
NuttX Apps: https://github.com/apache/nuttx-apps/tree/7a36a70e06402b9c1647dcf6833ca5a58e927132
# tools/configure.sh gd32f470zk-eval:nsh
# make -j
Memory region         Used Size  Region Size  %age Used
           flash:      143604 B         3 MB      4.57%
            sram:        6420 B       112 KB      5.60%
CP: nuttx.hex
CP: nuttx.bin

[lupyuen]

There are 2 extra locks, I'll remove them in the next 2 PRs.