Revert "[libc] Enable 'sscanf' on the GPU (#100211)"

Summary:
This fails tests in some situations, revert until it can be fixed.
This reverts commit 445bb35.

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Differential Revision: https://phabricator.intern.facebook.com/D60250606

Author: jhuber6

libc/config/gpu/entrypoints.txt

@@ -186,7 +186,6 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdio.sprintf
     libc.src.stdio.vsnprintf
     libc.src.stdio.vsprintf
-    libc.src.stdio.sscanf
     libc.src.stdio.feof
     libc.src.stdio.ferror
     libc.src.stdio.fflush

libc/docs/gpu/support.rst

@@ -234,11 +234,10 @@ printf         |check|    |check|
 vprintf        |check|    |check|
 fprintf        |check|    |check|
 vfprintf       |check|    |check|
-sprintf        |check|
-snprintf       |check|
-vsprintf       |check|
-vsnprintf      |check|
-sscanf         |check|
+sprintf        |check|    |check|
+snprintf       |check|    |check|
+vsprintf       |check|    |check|
+vsnprintf      |check|    |check|
 putchar        |check|    |check|
 fclose         |check|    |check|
 fopen          |check|    |check|