Avoid using sprockets for combining stylesheets

There are two options for combining stylesheets, using sprockets
(via `require` statements) or using sass (via `@import` statements).

This commit switches to using `@import` statements throughout, to
avoid a bug with libsass that prevents us from using bootstrap 5.2+

See sass/sassc-rails#174

Since R2 is not triggered when `@import`ing a file, we need to
switch the top-level stylesheet names to include the extension. We
can also take the opportunity to remove a level of indirection,
along with removing symlinks which cause problems on some platforms.

* openstreetmap#3984
* openstreetmap#3422

app/assets/stylesheets/print-ltr.scss

@@ -0,0 +1 @@
+@import "print";

app/assets/stylesheets/print-rtl.r2.scss

@@ -0,0 +1 @@
+@import "print";

app/assets/stylesheets/screen-ltr.scss

@@ -0,0 +1 @@
+@import "common";

app/assets/stylesheets/screen-rtl.r2.scss

@@ -0,0 +1 @@
+@import "common";