Add missing postinst script

[korydraughn]

Bug Report

The following lines ...

irods_capability_indexing/CMakeLists.txt

Lines 114 to 115 in 82eee54

	set(CPACK_DEBIAN_PACKAGE_CONTROL_EXTRA "${CMAKE_SOURCE_DIR}/packaging/postinst;")
	set(CPACK_RPM_POST_INSTALL_SCRIPT_FILE "${CMAKE_SOURCE_DIR}/packaging/postinst")

lead to the following output because the postinst script does not exist.

CPack Warning: Adding file to tar:                                                                                                                                                            
#top level directory: /home/kory/dev/github/irods_capability_indexing_build/_CPack_Packages/Linux/DEB/irods_capability-indexing-4.3.1.0-Linux/document-type                                   
#missing file: /home/kory/dev/github/irods_capability_indexing/packaging/postinst              
CPack Warning: Adding file to tar:                                                             
#top level directory: /home/kory/dev/github/irods_capability_indexing_build/_CPack_Packages/Linux/DEB/irods_capability-indexing-4.3.1.0-Linux/elasticsearch
#missing file: /home/kory/dev/github/irods_capability_indexing/packaging/postinst

Considering other iRODS repos contain that script, we should add it.

[korydraughn]

The references to the postinst script are not needed and should be removed.

See #131 (comment).

[korydraughn]

Related to #108.