Remove extra space by line numbers in Visual Studio 2015
First: Tools > Options > Text Editor > C# (or whatever language you use) > General > untick "Line numbers"
Second: Tools > Options > Text Editor > General > untick "selection margin" and "indicator margin"
There are following margins left of code: breakpoints/bookmarks (17 px), line numbers (35 px), quick actions (18 px), file changes (10 px) and outlining (9 px).
I don't see any deliberately wasted space and I don't see any public options to change width of existing elements.
If you are meaning this gutter between the line numbers and the gutter for break points, it how is used to display various code helpers (e.g. refactor indicators, lightbulbs for code improvements etc.)
If you take those indicators away you have about 2 extra character spaces.