Remove invalid escape char failing test

This test was never testing what it claimed to test, simply failing with
'ERROR: First statement must be a call to project' because it's missing
project().

Since #5279, all unrecognized escape sequences are literal, so I don't
think there's anything to test here.

test cases/failing/71 invalid escape char/meson.build

@@ -1,4 +0,0 @@
-# Make sure meson exits on invalid string
-# The string below contains an invalid unicode code point
-
-'my name is what \uxyzo who are you'