.gitignore: remove cross-compile *-libtool

This reverts commit dac90163a287 (".gitignore: add cross-compile
*-libtool") [1] merged in May 2022.

The old build system of OpenEmbedded used to rebuild and rename
plenty of tools to discriminate between host's and target's tools.
This creates, among others, the odd file "$CROSS_COMPILE-libtool"
that was addressed by the patch [1].
OpenEmbedded has dropped this odd behavior with patch [2], present
in OpenEmbedded 4.0 "kirkstone" tagged on April 2022.

In current situation:
- old OpenEmbedded use OpenOCD v0.11.0 or older, so will not use
  the patch [1];
- new OpenEmbedded has [2] applied, so will never take benefit
  from the patch [1].

As consequence, patch [1] is completely useless and keeping it in
OpenOCD can even be misleading.

Revert patch [1].

Change-Id: I75793fce82a5297d74af72e620c4e63cd5b15f90
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: https://review.openocd.org/6960 [1]
Fixes: dac90163a287 (".gitignore: add cross-compile *-libtool")
Link: https://git.openembedded.org/openembedded-core/commit/?id=4b308773eca7
Reviewed-on: https://review.openocd.org/c/openocd/+/7161
Tested-by: jenkins