COMP: Support Ninja build system.
COMP: Support Ninja build system.

Two INSTALL_COMMAND arguments were added to the superbuild external
project; with Ninja build system this generated an invalid command.
Removing the duplicate INSTALL_COMMAND fixes the issue.

Setting INSTALL_COMMAND to "" normally disables the install command
being generated; however when both arguments were present the
variable had the array value ["" ""], and so is still added to
build.ninja as "" "", which is invalid.

I'm not certain why the build still succeeded using generators other
than Ninja.

NIRALUser/SPHARM-PDM,bpaniagua/SPHARM-PDM,bpaniagua/SPHARM-PDM,NIRALUser/SPHARM-PDM,NIRALUser/SPHARM-PDM,bpaniagua/SPHARM-PDM