Remove the use of __LINE__

Fixes #10
To summarise: `__LINE__` reports its *own* line,
not the one the macro is invoked from.
RGBDS v0.6.0 deprecated it for that reason, removing this also prevents the warning.

Author: ISSOtm

structs.asm

@@ -25,7 +25,7 @@
 
 DEF STRUCTS_VERSION equs "3.0.0"
 MACRO structs_assert
-    assert (\1), "rgbds-structs {STRUCTS_VERSION} bug on line {d:__LINE__}. Please report at https://github.com/ISSOtm/rgbds-structs, and share your code there!"
+    assert (\1), "rgbds-structs {STRUCTS_VERSION} bug. Please report at https://github.com/ISSOtm/rgbds-structs, and share the above stack trace *and* your code there!"
 ENDM