exercises(rna-transcription): test: remove RnaError (#329)

We removed the RnaError tests [1][2], but didn't remove this
now-unused line from the test file.

[1] 3305bd3, 2023-02-17, "exercises(rna-transcription): remove unused testFailure function"
[2] 794420a, 2023-03-16, "exercises(rna-transcription): remove untested RnaError"

Author: ee7

exercises/practice/rna-transcription/test_rna_transcription.zig

@@ -3,7 +3,6 @@ const mem = std.mem;
 const testing = std.testing;
 
 const rna_transcription = @import("rna_transcription.zig");
-const RnaError = rna_transcription.RnaError;
 
 fn testTranscription(dna: []const u8, expected: []const u8) !void {
     const rna = try rna_transcription.toRna(testing.allocator, dna);