ARM: dts: msm: remove IPA keepalive voter for sdxprairie

This keepalive voter was added as a workaround to maintain
a persistent IPA vote while APPS is awake in order to
prevent an SMMU issue whereby the TCU may hang due to
an unresponding TBU under the IPA clock domain, let's
remove this workaround now that we're able to keep the
IPA clock on due to other means.

Change-Id: I841dea979e1d6676a9d6554c2c749f6ccf08eaa8
Signed-off-by: David Dai <daidavid1@codeaurora.org>