core: remove null check at executeReadList

I think that the null validation at executeReadList should be re examined
for the following reasons:
1) We already set the key RETURN_VALUE at createCallForRead before each
call, when creating the SimpleJdbcCall.
2) Until now executeRead called executeReadList and did not checked if
the returned list is null or not.

Change-Id: I54f25002701bcaa6e82a572b6fa5ed8ffa5e043a
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
core: Removing proxy method

executeReadAndReturnMap only acts as a proxy method to executeReadList,
therefore it can be removed.

Change-Id: I3079a893feaa49ef2b992c83fad97953fee21313
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>