doc: Remove GCC_ISSUES

This file is outdated, the mentioned issue is no longer there for
newer gcc versions - for bitcoin we haven't had to specify any special
optimization flag overrides for a long time (we compile using -O2).

doc/GCC_ISSUES

@@ -1,39 +0,0 @@
-# The Problem
-
-gcc sometimes generates slightly different code with the same semantics.
-
-## Optimizations
-
-The following optimizer flags reduce non-determinism when compiling wxWidgets:
-
-     -fno-tree-loop-optimize
-     -fno-trapping-math
-     -fno-tree-reassoc
-
-## Left Over
-
-Even with the above flags, the compiler still generates this difference in one out of 100 builds of wxWidgets:
-
-    monolib_property.o: elf64-elf_x86_64
-
-         aef7:    00
-         aef8:    45 31 f6                 xor     %r14d,%r14d
-         aefb:    48 8d 84 24 50 06 00 00  lea     0x650(%rsp),%rax
-    -    af03:    c6 44 24 67 00           movb    $0x0,0x67(%rsp)
-    -    af08:    48 83 c2 10              add     $0x10,%rdx
-    -    af0c:    48 83 c1 10              add     $0x10,%rcx
-    -    af10:    48 81 7c 24 08 ff ff ff  cmpq    $0xffffff,0x8(%rsp)
-    -    af18:    00
-    -    af19:    c6 44 24 47 00           movb    $0x0,0x47(%rsp)
-    -    af1e:    c7 44 24 24 00 00 00 00  movl    $0x0,0x24(%rsp)
-    +    af03:    c7 44 24 24 00 00 00 00  movl    $0x0,0x24(%rsp)
-    +    af0b:    48 83 c2 10              add     $0x10,%rdx
-    +    af0f:    48 83 c1 10              add     $0x10,%rcx
-    +    af13:    48 81 7c 24 08 ff ff ff  cmpq    $0xffffff,0x8(%rsp)
-    +    af1b:    00
-    +    af1c:    c6 44 24 47 00           movb    $0x0,0x47(%rsp)
-    +    af21:    c6 44 24 67 00           movb    $0x0,0x67(%rsp)
-         af26:    4c 8d bc 24 28 03 00 00  lea     0x328(%rsp),%r15
-         af2e:    48 89 44 24 10           mov     %rax,0x10(%rsp)
-         af33:    48 89 54 24 78           mov     %rdx,0x78(%rsp)
-