relocatable.texi: remove mention of OpenBSD?

[Reuben Thomas]

doc/relocatable.texi contains the following text:

 

 Also, installation with
@option{--enable-relocatable} might not work on OpenBSD, when the
package contains shared libraries and libtool versions 1.5.xx are used.

I checked, and the last version of OpenBSD to ship with libtool 1.5 was 5.0, support for which ended in November 2012. Perhaps this text can therefore be removed?

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