license.py: drop visit_Str

license.py: drop visit_Str

In https://git.openembedded.org/openembedded-core/commit/?h=kirkstone&id=abc93390a3f19bc4cc159c5690a478b9e2270906
visit_Constant was added for compatibility with Python 3.8 and newer, but
visit_Str was kept, because at that time bitbake did require Python 3.6.

Now latest bitbake requires 3.9 and even version 2.4 used in mickledore
builds requires 3.8 since:
https://git.openembedded.org/bitbake/commit/?h=2.4&id=744310f360d2288ac2ef07745abc86852126b5b9
so we can safely remove these.

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>