Resolve unused variable 'SECS_PER_LUMI' error

Clang 3.5.0 complains regarding unused variable:

    DQM/HLTEvF/plugins/TriggerRatesMonitor.cc:44:21: error: unused variable
    'SECS_PER_LUMI' [-Werror,-Wunused-const-variable]

The patch removes it.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Resolve all -Wunused-private-field warnings

A vast number of warnings in `DQM/HLTEvF` are

    private field '???' is not used [-Wunused-private-field]

It looks like a mistake of copy & paste. The patch removes all such
not needed private fields.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
std::fabs ==> std::abs if argument is int

We have `std::vector<int>`, thus std::abs must be used from `<cstdlib>`.
Otherwise it triggers unnecessary conversion to `double`.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #6359 from davidlt/clean-clang-DQM-HLTEvF

Clean up DQM/HLTEvF based on Clang warnings/errors