MAINTAINERS: Remove unused Qualcomm SoC mailing list

This patch removes the linux-soc mailing list from the Qualcomm SoC
entry.  We use the linux-msm and there is no need to have the second
one and this clears the list for use by others.

Signed-off-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>