Chore: Remove dead code related to billing (#935)

- remove subscription model
- remove billing-related code

app/javascript/dashboard/assets/scss/_woot.scss

@@ -3,7 +3,6 @@
 @import 'animations';
 
 @import 'foundation-custom';
-@import 'widgets/billing';
 @import 'widgets/buttons';
 @import 'widgets/conv-header';
 @import 'widgets/conversation-card';

app/javascript/dashboard/assets/scss/widgets/_billing.scss

@@ -1,75 +0,0 @@
-.billing {
-  @include full-height;
-
-  .row {
-    @include full-height;
-  }
-
-  .billing__stats {
-    @include flex;
-  }
-
-  .billing__form {
-    @include thin-border($color-border-light);
-    @include margin($zero - $space-micro);
-    @include full-height;
-    background: $color-white;
-
-    iframe {
-      @include full-height;
-      border: 0;
-      width: 100%;
-    }
-  }
-
-  .account-row {
-    @include padding($space-normal);
-    @include flex;
-    flex-direction: column;
-    // @include thin-border($color-border-light);
-    // @include margin(-$space-micro $zero);
-    background: $color-white;
-    font-size: $font-size-small;
-
-    .title {
-      color: $color-heading;
-      font-weight: $font-weight-medium;
-    }
-
-    .value {
-      font-size: $font-size-mega;
-      font-weight: $font-weight-light;
-      text-transform: capitalize;
-    }
-  }
-}
-
-.account-locked {
-  @include background-gray;
-  @include margin(0);
-}
-
-.lock-message {
-  @include flex;
-  @include full-height;
-  flex-direction: column;
-  @include flex-align(center, middle);
-
-  div {
-    @include flex;
-    @include full-height;
-    flex-direction: column;
-    @include flex-align(center, middle);
-
-    img {
-      @include margin($space-normal);
-      width: 10rem;
-    }
-
-    span {
-      font-size: $font-size-small;
-      font-weight: $font-weight-medium;
-      text-align: center;
-    }
-  }
-}