Revert "Increase maximum read-only mmap()s used from 1000 to 4096 on 64-bit systems"

After bitcoin/bitcoin#30039, the number of ldb files created is 16 times
smaller. 1000 files with the new default of 32MB is 32GB of database.

If we need more, we can increase the default file size again.

This patch seems unnecessary now.

This reverts commit 92ae82c.

Author: laanwj

util/env_posix.cc

@@ -42,8 +42,8 @@ namespace {
 // Set by EnvPosixTestHelper::SetReadOnlyMMapLimit() and MaxOpenFiles().
 int g_open_read_only_file_limit = -1;
 
-// Up to 4096 mmap regions for 64-bit binaries; none for 32-bit.
-constexpr const int kDefaultMmapLimit = (sizeof(void*) >= 8) ? 4096 : 0;
+// Up to 1000 mmap regions for 64-bit binaries; none for 32-bit.
+constexpr const int kDefaultMmapLimit = (sizeof(void*) >= 8) ? 1000 : 0;
 
 // Can be set using EnvPosixTestHelper::SetReadOnlyMMapLimit().
 int g_mmap_limit = kDefaultMmapLimit;