This reverts commit 5d1b8f41.

because since 306e439c manpages use troff aliases
instead of symlinks, so they should not be symlinked.

fix manpages in Debian

Matching libmariadb-dev package contents in official Debian repositories,
the MariaDB Connector/C files should go into a folder using the 'mariadb'
name. For compatibility with sources that expect to find 'mysql' stuff,
create a symlink from include/mysql to include/mariadb.

Using this package any source that expects MySQL Client libraries should
build without changes. Sources that expect MariaDB Connector C should
build using only libmariadb-dev.

Current MariaDB Connector/C does not build quite identically in
this source tree compared to how it build in Debian official.
The remaining differences are tracked in MDEV-14921.