`isbanded` should not be exported

[jishnub]

Since LinearAlgebra has an isbanded function as well, it's possible to confuse the two and encounter unexpected bugs. Perhaps it's best to not export BandedMatrices.isbanded in the next breaking release. This would also avoid clashes between the names if LinearAlgebra.isbanded is imported first.

julia> import LinearAlgebra: isbanded

julia> using BandedMatrices
WARNING: using BandedMatrices.isbanded in module Main conflicts with an existing identifier.

julia> @which isbanded
LinearAlgebra