configure.ac: remove an unnecessary libtool fix

configure.ac: remove an unnecessary libtool fix

This code was added in commit 75d393a563efb578c79364a277087c6326267f52
without explaining why. I assume it was a mistake, since it looks like
the libtool problem should have gone away a long time ago. The referenced
wiki page https://wiki.debian.org/RpathIssue says:

    Since libtool 1.5.2 (released 2004-01-25), on Linux libtool no
    longer sets RPATH for any directories in the dynamic linker search
    path, so this should no longer be an issue unless upstream used a
    really old version of libtool when creating their distribution
    tarball.

This code caused problems in OpenEmbedded, where the libtool script is
named "x86_64-oe-linux-libtool" or similar rather than just "libtool",
so the sed command failed with a file not found error. Rather than
adapting the code to OpenEmbedded's peculiarities, it seems best to just
remove the unnecessary code altogether.

Note: The rpath is set (hardcoded) for 'make' but it is corrected
for 'make install' by libtool.

Signed-off-by: Tanu Kaskinen <tanuk@iki.fi>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>