Revert from generic_norm2 to norm

[ferrolho]

This reverts commit 10de215.

As shown below, norm(a) is as precise as norm(Array(a)).

julia> using BenchmarkTools, LinearAlgebra, StaticArrays

julia> a = SA[0, 1e-180]
2-element SVector{2, Float64} with indices SOneTo(2):
 0.0
 1.0e-180

julia> norm(a)
1.0e-180

julia> norm(Array(a))
1.0e-180

julia> a
2-element SVector{2, Float64} with indices SOneTo(2):
 0.0
 1.0e-180

julia> @btime norm($(Ref(a))[]) # standard norm of static array
  2.500 ns (0 allocations: 0 bytes)
1.0e-180

julia> @btime norm($(Ref(Vector(a)))[]) # standard norm of array
  6.792 ns (0 allocations: 0 bytes)
1.0e-180

julia> @btime LinearAlgebra.generic_norm2($(Ref((a)))[]) # generic_norm2 of static array
  3.334 ns (0 allocations: 0 bytes)
1.0e-180

julia> @btime LinearAlgebra.generic_norm2($(Ref(Vector(a)))[]) # generic_norm2 of array
  6.666 ns (0 allocations: 0 bytes)
1.0e-180

Related to #622 and JuliaArrays/StaticArrays.jl#913.