core: Eliminated use of StringHelper.EqOp

Removed the method StringHelper.EqOp which was used mainly in searchbackend
module to check String equality with constants thus it could be replaced
with standard String implementation of equals and equalsIgnoreCase

Change-Id: Iaf3d65db824f31df05dc7b0eda5f9c24274cf53c
Signed-off-by: Tal Nisan <tnisan@redhat.com>