plugins/excelplugins: Remove erroneous .libs

The .libs directory is for internal usage by the libtool implementation only.

When build gnumeric with slibtool instead of GNU libtool this will be a failure where slibtool is far more strict about user errors.

rdlibtool: error logged in slbt_exec_link_create_dep_file(), line 1091: path not found: ../../src/.libs/.libs/libspreadsheet.a.disabled.

Full build log: gnumeric.slibtool.log

Gentoo Bug: https://bugs.gentoo.org/791610