Fix errors in Kotlin unit methods

Kotlin API fails on methods that don't return anything
(which currently just includes `delete` methods) because
the generated code by default tries to serialize the methods'
return types in all cases, even if the return type is `Unit`.
I'm not exactly sure whether this is properly a bug in the openapi
generator or in moshi. In any case, I was unable to fix the error
by doing what initially seemed most sensible, which was to register
a moshi adapter for the `Unit` type.

The fix here is not terribly elegant but in practice seems to fix
the issue. I will also see about opening/adding to an existing issue
on the openapi generator's github page and/or submitting a PR to
fix this upstream.

Note also that I've removed the `Serializer.kt` template, which
is unused. We've already got an actual `Serializer` class
stored in the generated code folder, so there's no reason to
have the template.

Fixes #1117.

kotlin/lib/src/test/com/svix/kotlin/BasicTest.kt

@@ -24,6 +24,7 @@ class BasicTest {
                     )
                 )
             )
+            svix.application.delete(applicationOut.id)
         }
     }
 

kotlin/templates/libraries/jvm-okhttp/infrastructure/ApiClient.kt.mustache

@@ -171,9 +171,18 @@ import kotlin.random.Random
             )
             {{/jvm-okhttp3}}
             {{#jvm-okhttp4}}
-            mediaType == JsonMediaType -> {{#moshi}}Serializer.moshi.adapter(T::class.java).toJson(content){{/moshi}}{{#gson}}Serializer.gson.toJson(content, T::class.java){{/gson}}{{#jackson}}Serializer.jacksonObjectMapper.writeValueAsString(content){{/jackson}}{{#kotlinx_serialization}}Serializer.jvmJson.encodeToString(content){{/kotlinx_serialization}}.toRequestBody(
-                mediaType.toMediaTypeOrNull()
-            )
+            mediaType == JsonMediaType -> {
+                // This is a bit of a hack to avoid moshi
+                // failing on trying to call `toJson` on
+                // a Unit type:
+                if (T::class.java == Unit::class.java) {
+                    ByteArray(0).toRequestBody()
+                } else {
+                    {{#moshi}}Serializer.moshi.adapter(T::class.java).toJson(content){{/moshi}}{{#gson}}Serializer.gson.toJson(content, T::class.java){{/gson}}{{#jackson}}Serializer.jacksonObjectMapper.writeValueAsString(content){{/jackson}}{{#kotlinx_serialization}}Serializer.jvmJson.encodeToString(content){{/kotlinx_serialization}}.toRequestBody(
+                        mediaType.toMediaTypeOrNull()
+                    )
+                }
+            }
             {{/jvm-okhttp4}}
             mediaType == XmlMediaType -> throw UnsupportedOperationException("xml not currently supported.")
             // TODO: this should be extended with other serializers