Issue #7498 Remove !global flag from $grid-column-count

Creating a custom row size using the grid-row mixin was not properly generating the correct CSS. Removing the !global flag from the $grid-column-count in the grid-context mixin seems to correct this.

Tested using following scss. The first list generates a row 17 columns wide, while the second list generates the default 12
ul.one
  @include grid-row(17)
  li
    @include grid-column(1)

ul.two
  li
    @include grid-column(1)