small improvements in getOffset method

Hi Tony,

I suggest to simplify a little the code of `getOffset` function. Additionally it will be removed dependency from `cellLayout` and `$.jgrid.cellWidth` from the code.

By the way the input parameter of  `getOffset` function (`iCol`) will be get from the previous call of `getColumnHeaderIndex` (see [here](https://github.com/tonytomov/jqGrid/blob/v4.4.4/js/grid.base.js#L2371-L2373)) which uses internally `ts.grid.headers[i].el` expression in the loop (see [the source code](https://github.com/tonytomov/jqGrid/blob/v4.4.4/js/grid.base.js#L2166-L2175)). So there are no need to include in code of `getOffset` function any validation whether the expression `$th = $(ts.grid.headers[iCol].el)` from the suggested code is correct or not. It's always valid.

Best regards
Oleg