Investigate removing xmpsdk from code base

[clanmills]

Exiv2 has been shipping with a copy of the Adobe XMPsdk for a very long time. We don't include the code of any other dependency (expat, libz, libiconv and others). I think the reason is because the early versions of exiv2 had no CMake support and XMPsdk has always exclusively used CMake.

Luis added support to build and link more modern versions of XMPsdk to v0.27 for several reasons:

1. Several users requested that we use XMPsdk as an external library because they wanted to use it in their applications. Our 2008 edition of XMPsdk was giving them link problems.
2. We're stuck in a time-warp with this elderly version of XMPsdk.

[hassec]

I'm adding the INI parser to this list of things to remove which is currently also just dropped into the exiv2 source tree

[clanmills]

I've opened a new issue concerning src/ini.cpp #1811.

I think we should restore the title of this issue to Investigate removing xmpsdk from code base because those issues are different.

[kevinbackhouse]

I noticed that the XMPsdk is on GitHub now: https://github.com/adobe/XMP-Toolkit-SDK
So maybe we can include it as a sub-module.