package/rtmpdump: Remove optional dependency to PolarSSL

package/rtmpdump: Remove optional dependency to PolarSSL

The makefile of rtmpdump has no support for PolarSSL, so lets remove
support for it, as supposed by Gustavo:
http://article.gmane.org/gmane.comp.lib.uclibc.buildroot/129531

Fixes
http://autobuild.buildroot.net/results/45d/45dd9adbac449ce0ed66af5b4655b4d9b37faa62/
http://autobuild.buildroot.net/results/ae0/ae0c4bab7975ed2ad77a9f9fd6a300d1327d56b9/
http://autobuild.buildroot.net/results/029/029695a022ba12ff177521ebc3334d7ac2373ddc/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>